1. Field of the Invention
This invention relates to the preparation of polymers, and specifically to an improved process for the polymerization and isolation of methyl methacrylate polymer powders that are especially suitable for dispersion in nonaqueous media to form plastisols and organosols.
2. Description of the Prior Art
Certain polymeric resin dispersions referred to as plastisols are fluid mixtures, ranging in viscosity from pourable liquids to heavy pastes, obtained by dispersing polymeric resin particles in nonvolatile, nonaqueous liquid plasticizers, i.e., materials which are compatible with the polymer or resin and increase its workability and flexibility but are not solvents for the polymer or resin under ordinary conditions of storage. When the plastisol has been formed into a desired shape, e.g., by molding or coating, it can be heated to coalesce the resin or polymer particles and the nonvolatile liquid constituent, thereby forming a homogeneous mass. Volatile diluents can be added to plastisol dispersions to modify their viscosity and to achieve desirable handling characteristics in coating or other forming operations. When the dispersion contains no more than 10% volatile diluent, it is still regarded as a plastisol; when the volatile diluent content exceeds 10%, the dispersion is regarded as an "organosol", H. A. Sarvetnick, "Plastisols and Organosols", Van Nostrand Reinhold Company, New York, New York (1972), page 201.
Theoretically, by appropriate choice of ingredients, any polymeric resin can be made into a plastisol. In practice, however, both the commercial use and the technical literature have focused almost exclusively on the use of polyvinyl chloride in forming plastisols, to the extent that "plastisol" is cross-referenced to "polyvinyl chloride" in Chemical Abstracts and in many textbooks plastisols are described as suspensions of polyvinyl chloride resin.
The pre-eminence of polyvinyl chloride in the practical and technical plastisol art results from the fact that many polymers with otherwise highly useful and desirable characteristics can be dispersed in suitable plasticizers only with great difficulty to give dispersions that have impractically high initial viscosities or very short storage life or both. Among these other desirable materials are various polymers and copolymers of methyl methacrylate, which are attractive for a wide variety of uses because of their clarity, toughness, durability, availability, relatively low cost, and ease of forming into desired final products by a number of methods. Despite their many desirable attributes, these polymers and copolymers have found no practical application in the plastisol and organosol arts because it has not been known heretofore how to prepare compositions with suitably low and stable viscosities.
It has now been found that plastisols and organosols with very acceptable rheological properties can be prepared from methyl methacrylate polymer and copolymer powders that have been (a) made by stepwise emulsion polymerization in aqueous media in the absence of emulsifiers and surfactants and in such a manner that most of the monomer added at each step is consumed before the addition of the succeeding portion, and (b) isolated by evaporation of the latex at a temperature that is at least 30.degree. C., preferably 40.degree.-50.degree. C., below the glass transition temperature of the polymer. Particularly advantageous is the use of a stepwise polymerization procedure that entails the presence of "seed" polymer in the aqueous emulsion.
Emulsion polymerization, including stepwise and "seed" polymerization, of methyl methacrylate polymers and copolymers in aqueous media with the help of emulsifiers and surfactants is known. It is also known to use emulsifiers and surfactants when making polyvinyl chloride powders for use in plastisols. In the known plastisol art, based on polyvinyl chloride, it is common practice to dry the polymer powder at a temperature sufficiently high to produce a thin surface skin of fused resin, i.e., at a temperature higher than the glass transition temperature of the polymer, e.g., C. E. Schildknecht, "Polymer Processes", High Polymers, Volume X, Interscience Publishers, Inc., New York (1956), page 558. A two-stage drying procedure for polyvinyl chloride is known, wherein 88-97% of the water is removed in a first step at a temperature higher than the glass transition temperature of the polymer and the remaining water is removed in a second step simultaneously with grinding at a temperature below the glass transition temperature. In contrast to, and simplification of, these prior art practices, the improved process of the present invention omits emulsifiers and surfactants previously thought to be required and accomplishes the drying in a single step at a temperature substantially below the glass transition temperature of the methyl methacrylate polymer or copolymer.